1. Technical Field
The present invention relates generally to a composite substrate, a semiconductor device which includes the composite substrate, and a method of manufacturing the composite substrate; the present invention relates more particularly to a composite substrate with a high quality nitride-based single crystal layer, a semiconductor device which includes the composite substrate, wherein the semiconductor device can be, for example, a nitride-based semiconductor light-emitting device, a high electron mobility transistor (HEMT), a metal-oxide-semiconductor field-effect-transistor (MOSFET), a laser diode, etc., and a method especially for manufacturing the nitride-based semiconductor light-emitting device of which a nitride-based multi-layer structure on the composite substrate has lower defect density, and a turn-on voltage less than 2.9 V while provided with an input current of 20 mA.
2. Description of Related Art
A semiconductor device such as a semiconductor light-emitting device, a HEMT, a MOSFET, a laser diode, etc., is typically has a buffer layer provided therein to improve the quality of the subsequent epitaxy and the device itself. The related technique is conventional, for the prior art is referred to in U.S. Pat. No. 7,834,343, No. 7,666,765, and Taiwan patent publication No. 201331987.
Take a semiconductor light-emitting device for example, with a buffer layer provided between a crystal substrate and a multi-layer light-emitting structure thereof, the problem of lattice mismatch between the multi-layer light-emitting structure and the crystal substrate can be eased, and the defect density of the light-emitting layer, especially the density of threading dislocations, can be reduced as well. Furthermore, the buffer layer also contributes to decrease the thermal expansion coefficient difference between the multi-layer light-emitting structure and the crystal substrate.
In recent years, nitride-based semiconductor materials are widely used in light-emitting devices of blue waveband. A conventional nitride-based semiconductor light-emitting device typically has a sapphire substrate, and has a nitride buffer layer, which typically is a GaN/AlN buffer layer, provided between the sapphire substrate and a nitride-based light-emitting layer. For one skilled in the art, it is well-known that the quality of the nitride buffer layer would directly affect the optical-electrical property of the nitride-based semiconductor light-emitting device. With the current developed metal-organic chemical vapor deposition (MOCVD) process, so-called low temperature MOCVD GaN/AlN buffer layer can be produced at a low temperature between 400 and 800 degrees Celsius. However, there is still much room for improvement in the quality of a low temperature MOCVD GaN/AlN buffer layer, for the turn-on voltage of a nitride-based semiconductor light-emitting device which is made by solely using a low temperature MOCVD GaN buffer layer is higher than 2.9 V, which is commonly considered too high.